Uncovering the mechanism of bioleaching of enargite by XPS.

Biooxidation processes (oxidation mediated by microorganisms) of sulfide minerals are particularly suitable for low-grade ores and are more environmentally friendly compared to conventional physicochemical mineral beneficiation processes. Along with economic interest, oxidative dissolution mediated by microorganisms is of environmental concern since the latter are usually present in AMD (Acid Mine Drainage) and they play a decisive role in the release of toxic elements into the environment. According to the integral model for bioleaching [1], Fe (III) and H+ are the only responsible for sulfides dissolution and the role of bacteria is to catalyze Fe (III) and H+ regeneration and to concentrate such oxidants at the mineral surfaces by extra-cellular polymeric substances (EPS) that mediate the bioleaching process. EPS is produced by the living bacteria and forms kind of a bridge between the cell and the mineral surface. As the EPS layer is able to complex iron(III) ions into glucuronic acid-iron complexes the actual concentration of iron(III) ions on the mineral surface may well be higher than the concentration measured in solution. This would increase the electrochemical potential of the mineral surfaces resulting in the higher dissolution rate observed in the presence of bacteria. In this work the surface characterization by X-ray photoelectron spectroscopy (XPS) analyses of the exopolymeric layer formed during biooxidation of enargite (Cu3AsS4) by a strain of Acidithiobacillus ferrooxidans adapted to arsenic is presented. Enargite solubility was highly enhanced by A. Ferrooxidans: solution analyses showed that Cu and As dissolved stoichiometrically with a dissolution rate of about 3 - 5 times higher compared to the abiotic control. XPS spectra of C1s, O1s, N1s and P2p regions acquired on bacteria cells and on bioleached mineral grains were processed so that the surface composition of the EPS layer was obtained. The fraction of proteins, hydrocarbon and polysaccharides of the EPS was estimated and followed upon time up to 22 weeks. It was observed that the thickness of organic layer at the mineral surfaces increases with bioleaching time from 1 nm (after 44 days) to 2.5 nm (after 157 days) as the protein content while the hydrocarbon and polysaccharides decreases with leaching time. References 1. W Sand, T Gehrke, P Josza, A Schipper, (bio)chemistry of bacterial leaching – direct vs. indirect bioleaching, Hydrometallurgy 59 (2001) 159 - 175